The present invention is directed to the field of compressors, and is particularly directed to reciprocating compressors.
There are three types of capacity controls that are common to reciprocating and other positive-displacement compressors. In a smaller compressor, a pressure switch is utilized to start and stop the motor in response to changes in discharge pressure. In a medium size compressor a constant speed control is often used in combination with the pressure switch. Constant speed control may be accomplished by throttling the intake of the compressor. Other capacity control techniques which involve changing the clearance volume or modifying the port timing of the compressor are also in use for rotary compressors. Large reciprocating compressors use capacity variation techniques based on disabling the compression process by opening the cylinder inlet or outlet valves. For a compressor driven by a variable-speed motor or engine, the speed of the motor or engine can be varied to control the capacity of the compressor.
The technique of throttling the intake has not been applicable to lubricated reciprocating compressors, which use one or more pistons to drive a compressed gas flow. By throttling the intake, the gas pressure at the top of the piston would be lower than the crankcase pressure, which could allow oil to migrate from the crankcase to the top of the piston. Such migrating oil could become entrained into the compressed gas.
In order to prevent pressure buildup inside the lubricant chamber, all reciprocating compressors are equipped with a vent or breather system. Some reciprocating compressors have a vent that is connected to the inlet plenum by means of a conduit which may include a check valve. Other compressors have a vent, with or without a check valve, that is open to the surrounding atmosphere.